Investment casting processes are widely used to create hollow, near net-shape metal components, e.g. turbine blades, by pouring molten metal into a ceramic mould of the desired final shape and subsequently removing the ceramic. The process is an evolution of the lost-wax process, wherein a component of the size and shape required in metal is manufactured in wax using wax injection moulding, which pattern is then dipped in ceramic slurry to create a shell; the wax is then removed and the shell fired in order to harden it. The resulting shell thus has one or more open cavities therewithin for receiving molten metal when poured inside, the cavities being of the size and shape required for the final component, e.g. a turbine blade.
Often engine parts are required to have complex internal cavities for the purpose of acting as internal cooling channels. To form such internal cavities, at least one, and often several, ceramic core(s) is/are required in order to define and form the internal channels during the casting process. The cores are manufactured separately and placed inside the wax pattern die prior to wax injection. After casting the alloy the cores are leached out with alkaline solution to leave the hollow metal component.
Ceramic cores are usually manufactured by particle injection moulding (PIM). A ceramic material, usually silica, is suspended in an organic binder (vehicle) to create a feedstock, which is then injected into a die cavity of the required size and shape to create a “green” component comprising the ceramic and binder. The binder is subsequently thermally or chemically removed and the ceramic consolidated by sintering at elevated temperatures, to give the final ceramic core.
Recently advanced cooling concepts applied to turbine blades for use in gas turbine engines often require a complex configuration of core passages to give the most efficient level of cooling on the final component. One design that can give a large improvement in cooling is known as a contraflow-cooled component, as disclosed for example in U.S. Pat. No. 7,442,008B2. An example of such a turbine blade is shown schematically in FIG. 1 of the accompanying drawings. In this design of blade a main core body A and a secondary core body B, each comprising a desired arrangement of cooling air passages 10, are pre-manufactured separately and subsequently assembled together. In the final component this allows cooling air to be passed up certain selected ones of the internal passages and down other selected ones of the passages, in the manner of a contraflow.
Thus, and since a wax pattern for forming the complete core arrangement corresponding to this design of blade cannot be injection moulded as a single piece, it is moulded in two separate pieces which are then assembled together using a fixture to hold the cores in the correct relative positions to the required accuracy during the relevant stages of the casting process. In practice the cores are held together using a fugitive material compatible with the wax injection moulding process and which is able to be removed using the same processes as for a standard investment casting component.
It is known in the art to use so-called “bumpers”, or protrusions, on the exterior walls or faces of ceramic cores for controlling integral wall thicknesses of an investment cast component. Examples of such bumpers and their use are disclosed in US2002/0129924A1 and U.S. Pat. No. 5,296,308. However, such bumpers have limited utility in some situations.
Typically, thin or narrow core passages require support in order to maintain their dimensional stability. At the root and tip ends of the blade to be cast this may be readily done via contact with the casting shell. However, according to current practices, intermediate these locations this supporting must be done using thin ceramic pieces which are not required by the blade design but are purely for supporting the core(s) during subsequent processing. This however requires a compromise between design and manufacturing optimisation, as these extra pieces are often detrimental to the cooling effectiveness of the final component. Moreover, for the two-piece assembled core method such as described above and shown in FIG. 1, support between cores A and B of this type is not possible.
This issue may be partially solved through the use of bumpers to maintain desired relative spatial positionings between cores and minimum cast wall thicknesses, especially at particular locations which may be prone to movement or distortion during various stages of casting and onward processing, as disclosed for example in U.S. Pat. No. 5,296,308 mentioned above. Here a series of e.g. circular cross-sectioned bumpers (protrusions) on the outer walls of the various cores engage with an inner wall of an outer ceramic shell mould, leading to the external walls of a resulting cast component being maintained at a desired minimum thickness, despite movement or distortion of the cores' geometries possibly occurring during the casting process.
An arrangement of bumpers of the type disclosed in U.S. Pat. No. 5,296,308 might in principle be considered for application to the assembly of the two-body core arrangement shown in FIG. 1. Doing that may be expected in principle to result in an arrangement of cores 10, regions 20 between the cores corresponding to resulting internal cast walls, with bumpers 30 on the relevant ones of the cores 10, as shown in FIG. 2 of the accompanying drawings. Here, if either of the core bodies A, B moves relatively towards the other during the casting process, then the relevant bumper on the wall of core body B will substantially prevent the relevant core passage(s) from collapsing beyond a minimum allowable amount. However in this case that minimum allowed amount may often turn out to be substantially 0 mm, owing to the bumpers' abutment against the wall of core body A, and thus through-holes may occur in the resulting cast wall at the sites of the respective bumpers 30. The presence of such holes is undesirable, since they may compromise the cooling efficiency of the arrangement of cooling air passages, especially in a contraflow-cooled turbine blade as discussed above.
Even if it were to be considered to manufacture core body B such that its bumpers would not abut the wall of the opposing core body A, and instead would leave a small gap of e.g. of the order of 0.1-0.2 mm therebetween (so that the cast walls between the core bodies A, B would remain intact), then it might be expected that that would result in a casting that would be acceptable for end use as a gas turbine engine turbine blade. However, in practice this is not necessarily so.
Instead, the application of the disclosure of U.S. Pat. No. 5,296,308 to turbine blades having interior cast walls presents a new problem: when used on internal passageways defined by internal cast walls such as those proposed in relation to FIG. 1, which are required to carry most of the blade load when in use, the variable nature of the overall cast surface can cause extremely large stresses to be present in particular locations, particularly at the potential break-through point adjacent the head of each bumper. This may well render the resulting blade unsuitable for use in an engine. Thus it not possible simply to apply the known teaching of the use of bumpers in a known manner to the casting of certain walls of many engine parts, especially turbine blades.
WO00/78480 describes a multi-wall ceramic core assembly and method of making same wherein a plurality of individual thin wall, arcuate (e.g. airfoil shaped) core elements are formed in respective master dies to have close tolerance mating locating features that substantially prevent penetration of molten metal between the interlocked features during casting, the individual core elements are fired on ceramic supports to have integral locating features, the prefired core elements are assembled together using the locator features of adjacent core elements, and the assembled core elements are held together using a fugitive material. The multi-wall ceramic core assembly so produced comprises the plurality of spaced apart thin wall, arcuate core elements and located by the mated close tolerance locating features.